In recent years, in order to cope with a drastic increase of communication traffic with popularization of the Internet, in a WDM optical transmission system, enlargement of transmission capacity due to high bit rate of optical signals and high density of wavelength channels has been strongly demanded. Further, as illustrated in FIG. 13 for example, there has been progressed the establishment of all optical network which connects in seamless between a core network and a metro network or an access network by utilizing a wavelength selective switch (WSS), to flexibly administrate optical signals for each wavelength (refer to: Glenn Baxter et al., “Highly Programmable Wavelength Selective Switch Based on Liquid Crystal on Silicon Switching Elements”, Optical Fiber Communication Conference (OFC) 2006, OTuF2; and Steven Frisken, “Advances in Liquid Crystal on Silicon Wavelength Selective Switching”, Optical Fiber Communication Conference (OFC) 2007, OWV4).
Typically, a wavelength bandwidth (spectrum width) of optical signal of each wavelength contained in a WDM light is proportional to a bit rate (modulation frequency) thereof. Therefore, as the bit rate of the optical signal of each wavelength is increased, the spectrum width thereof becomes broader, and accordingly, it becomes hard to wavelength multiplex each optical signal in high density. Further, in the optical network as illustrated in FIG. 13, since the spectrum width of the optical signal is limited by an optical device having a filter function which is arranged in each node or the like on a transmission path, transmission characteristics are degraded. As the optical device having the filter function (to be referred to as a band-limiting device hereunder), there are a multiplexer (MUX) which multiplexes optical signals of respective wavelengths to generate a WDM light, a demultiplexer (DEMUX) which demultiplexes the WDM light into the optical signals of respective wavelengths, an optical switch which is used for processing of optical adding/dropping or optical cross-connecting, and the like.
As a conventional technology for decreasing an influence of the above band-limiting device to enable the realization of higher density transmission of WDM light, there has been proposed a partial DPSK (differential phase shift keying) format as disclosed in U.S. Patent Application Publication No. 2007-0196110. This PDPSK format is a technology for, in a receiving section of an optical transmission system in which DPSK modulated optical signals are received/transmitted, setting a delay amount in a delay interferometer to be shorter than 1 bit time, to suppress characteristic degradation of the optical signals passed through the band-limiting device on the transmission path. It has been reported that, by adopting such a PDPSK format, the transmission of the WDM light obtained by wavelength division multiplexing the optical signals of 40 gigabit/second (Gb/s) at 50 gigaherz (GHz) spacing can be realized (refer to B. Mikkelsen et al., “Partial DPSK with excellent filter tolerance and OSNR sensitivity”, Electronics Letters, 2006, Vol. 42, No. 23).
However, even in the WDM optical transmission system adopting the above PDPSK format, there is a problem in that degradation of the transmission characteristics is inevitable if the limitation of the spectrum width of each optical signal by the band-limiting device on the transmission path becomes more strictly. Hereunder, there will be described this problem in detail.
FIG. 14 illustrates a typical spectrum of DPSK modulated optical signal of 40 Gb/s. As illustrated in FIG. 14, the spectrum of DPSK modulated optical signal has sidebands on a high frequency side and on a low frequency side around a frequency (wavelength) of a carrier wave (an output light from a laser light source), and in the case of 40 Gb/s exemplarily illustrated herein, the DPSK modulated optical signal has the bandwidth of equal to or higher than 100 GHz in total. The WDM light is obtained by wavelength division multiplexing a plurality of the above optical signals of which center wavelengths are different from each other. In the transmission of such a WDM light, even if the optical signals of respective wavelengths are at high bit rate such as 40 Gb/s, the signal transmission by dense wavelength multiplexing at 100 GHz spacing or 50 GHz spacing is becoming typical. In the case where such a high-speed and high-dense WDM light is transmitted, there is a problem in that in the band-limiting device arranged on the transmission path, sideband components of the optical signal of each wavelength are eliminated so that the transmission characteristics are degraded.
To be specific, in the case where the DPSK optical signal of 40 Gb/s illustrated in FIG. 14 passes through a primary Gaussian filter (band-pass filter) having 3 dB bandwidth of 40 GHz as illustrated in FIG. 15 for example, the spectrum of this optical signal has a form as illustrated in FIG. 16. As apparent from FIG. 16, the sideband components of the optical signal are attenuated during the optical signal passes through the band-pass filter, so that a spectrum form thereof is significantly changed. The above described band-limiting device on the transmission path is equivalent to the band-pass filter in this example.
Here, there will be described in detail an influence of the attenuation of sideband components of the optical signal as described above on the transmission characteristics, using a simulation result for the case of adopting the above PDPSK format.
FIG. 17 is a diagram illustrating the outline of calculation model in the simulation relating to the WDM optical transmission system adopting the PDPSK format. In this calculation model, a pseudo random bit stream generated in a PRBS circuit 111 inside of an optical transmitting section 110 is fed to a DPSK pre-coder 112 so that a data signal corresponding to a DPSK format is generated, and a LN modulator (MOD) 112 is driven based on a modulation signal generated in a driver circuit (DRV) 113 in accordance with the data signal. As a result, a carrier wave output from a laser light source (LD) 115 is phase modulated so that a DPSK optical signal is transmitted to a transmission path 120. On the transmission path 120, there are arranged: an optical attenuator (ATT) 131 and an optical amplifier 132, for controlling an optical signal to noise ratio (OSNR) of the DPSK optical signal; and a band-pass filter (primary Gaussian filter) 133 for limiting the spectrum width of the DPSK optical signal, and the DPSK optical signal passed through the band-pass filter 133 is received by an optical receiving section 140. In the optical receiving section 140, the DPSK optical signal is processed to be demodulated by a delay interferometer 141 and a balance receiver 142, so that a bit error rate (BER) of the received signal is measured by a BER measuring device 143. A delay time in the delay interferometer 141 is set at 16.25 picoseconds (ps) corresponding to 65% of 1 bit time, to thereby correspond to the PDPSK format.
FIG. 18 illustrates relations between the OSNR and the BER for the cases where the 3 dB bandwidth of the band-pass filter 133 is 37 GHz, 26 GHz and 21 GHz, in the calculation model in FIG. 17. In this regard, the OSNR is a value measured with the resolution 0.5 nm, and the bands of the optical transmitting section 110 and optical receiving section 140 are properly set. As apparent from FIG. 18, when the 3 dB bandwidth of the band-pass filter 133 is broad to an extent of 37 GHz, a value of the BER is significantly decreased as the OSNR is increased (improved). On the other hand, if the 3 dB bandwidth of the band-pass filter 133 becomes narrower to 26 GHz or 21 GHz, the value of the BER is not so decreased although the OSNR is increased. Namely, even in the case of adopting the PDPSK format, as the bandwidth of the band-pass filter 133 is decreased, the BER is degraded.
Accordingly, in the case where the band-limiting device of narrow bandwidth is arranged on the transmission path, or in the case where a large number of band-limiting devices is arranged on the transmission path and the bandwidth of total filter characteristics of these band-limiting devices becomes narrower, even if the PDPSK format is adopted, it becomes hard to effectively suppress the degradation of transmission characteristics caused by the attenuation of sideband components by the band-limiting device. Such a problem caused by the attenuation of sideband components is not limited to the DPSK format for optical signal modulation, but also, is common to phase modulating formats other than the DPSK format, intensity modulating formats such as a NRZ (non return to zero) format, a RZ (return to zero) format and the like.